This application claims priority of German patent application number 100 25 264.8, filed May 22, 2000.
The present invention concerns a field-effect transistor based on embedded cluster or island structures made of semiconductor material and a process for its production. The invention refers both to those field-effect transistors in which the strain caused by cluster structures is used in an immediately adjacent channel region and to those field-effect transistors in which the channel region itself is formed by the cluster structures.
Already, field-effect transistors are known in which the channel region is formed from strained semiconductor material. In the publication xe2x80x9cElectronic Mobility Enhancement in Strained-Si N-Type Metal-Oxide-Semiconductor Field-Effect Transistorsxe2x80x9d by J. Welser et al., a field-effect transistor is described in which a pseudomorphically strained Si-channel region is produced by growing a Si layer on a relaxed SiGe buffer layer. The expansion strain in the Si material of the channel region results in a change in the band structure and in a cancellation of the six-fold band degeneracy, whereby higher electron and hole nobilities may be obtained.
The disadvantages of this type of strained field-effect transistors consist, however, in that the relaxed SiGe buffer layer is a few microns thick and is thus expensive and time-consuming to produce. Moreover, these layers have a high concentration of crystal defects which are an impairment to the large-area integration of transistors.
EP 0,921,575 describes a heterostructure MIS field-effect transistor with an expansion strained channel layer whereby a first Si layer, a carbon-containing Si2xe2x88x92YCY layer which serves as a canal region, and an optional second Si layer are deposited in this order on a substrate. The carbon content Y and the thickness of the Si2xe2x88x92YCY layer are selected such that the carbon-containing Si material has an expansion strain and this results, in the same manner as in pure expansion-strained silicon, in a splitting of the conduction and valence bands, reduced effective masses, and an increase in the mobility of charge carriers. Expansion-strained field-effect transistors of this type are, obviously, simpler to produce, since no thick SiGe buffer layers have to be grown. The disadvantage of these transistors lies, however, in that their most important region, i.e., the channel region, is no longer formed from pure silicon, but from carbon-containing silicon, whereby the component characteristics are extremely dependent upon the carbon content and the thickness of the channel layer. Thus, it is difficult to produce these field-effect transistors with reproducible component characteristics.
The object of the present invention is to report a fundamentally new material structure for a field-effect transistor, especially for its channel region, by means of which the performance characteristics of the transistor can be improved. A further object of the invention is to report a process for its production.
A field-effect transistor according to the invention, also referred to as a DOTFET, is essentially based on the use of clusterlike or islandlike semiconductor material structures in the channel region or in the vicinity of the channel region of the field-effect transistor. The clusters may be used in two fundamentally different embodiments. According to a first embodiment, they can be arranged below the actual channel region and generate a strain field in the semiconductor material of the channel region, whereby the band structure of the semiconductor material is suitably altered. The band structure may be altered with suitable material selection such that the effective masses of the charge carriers can be reduced and their mobility can be increased. In this variant, the clusterlike structures are, however, not themselves a part of the channel region of the transistor. According to a second embodiment, the channel region itself may also be formed by the clusters or islands. Mixed forms between these two embodiments are also conceivable, wherein the source/drain current flows both through the clusters and through semiconductor layers strained by the clusters.
The field-effect transistor also has source and drain regions which run vertically at least to the channel region. In the above-described first embodiment, wherein the channel region is formed by an expansion-strained semiconductor layer formed above a cluster layer, it is advantageous that relatively flat source/drain regions are formed. However, if, according to the first embodiment a plurality of the embedded expansion-strained silicon channel regions are to be generated and used, the source/drain regions should be formed to a greater depth, possibly down to the lowest semiconductor channel layer. If the channel region according to the second embodiment is to be formed by the clusters themselves, it may be necessary that the source/drain regions be formed to at least the one cluster layer or with the use of a plurality of cluster layers to a greater depth, possibly down to the lowest cluster layer. However, this is not absolutely obligatory even with the second embodiment since the channel region can be formed uniformly from the clusters of the second semiconductor material and the surrounding first semiconductor material located above it, whereby it is then adequate if the source/drain regions only extend to the semiconductor layer located above them. Above this, the embedded island structures of the second semiconductor material can form potential cavity structures within the first semiconductor material because of a lower band gap.
An essential characteristic of the field-effect transistor according to the invention consists in that at least part of the clusters run laterally between two sections which lie either, according to the first embodiment, directly under the source/drain regions, or, according to the second embodiment, inside the source/drain regions. In the first embodiment, it is, consequently, ensured that the semiconductor channel region expansion-strained by such clusters is formed between the source and drain regions. In the second embodiment, it is guaranteed that the channel region formed by the clusters themselves is formed between the source and drain regions.
In the production of field-effect transistors according to the invention, use is made of the phenomenon that with lattice-mismatched growth of a semiconductor material on a substrate of another semiconductor material with suitable selection of material, self-organized growth of island structures on the substrate occurs according to the Stranski-Krastanov mechanism. The layer, which is applied flat, breaks down spontaneously into a three-dimensional system of clusters because the elastic energy stored in the entire system is reduced thereby and an optimum compromise between different energies partially canceling each other occurs. In EP 0,437,385 A, for example, the growth of InAs growth islands on a GaAs substrate is described. Because of the lattice mismatch, a thin InAs wetting layer initially forms, above which a further growth of InAs results in the spontaneous formation of microscopic island structures. Under certain growth conditions, the exact same behavior is observed in the growth of germanium-rich SiGe on silicon substrates. For the present invention, use is made of the fact that the islands on the surface partially relax elastically. When the semiconductor material of the substrate is again grown on such growth islands, i.e., for example, silicon material on SiGe islands, a strain field is generated in this material. Since SiGe has a higher lattice constant than Si, the silicon material exhibits an expansion strain above the SiGe islands. Such an expansion-strained silicon layer can thus be generated by growth on SiGe islands, without having to produce a multi-micron-thick, relaxed, high-dislocation SiGe layer.
For a production process of field-effect transistors according to the invention, in particular in relatively high numbers and regular arrangement, it is thus advantageous if the growth of the islands proceeds not completely self-organized but, instead, is oriented by suitable specimen preparation. In a preferred method of producing a field-effect transistor, the clusters are, for example, deliberately produced spatially on prestructured substrates such that the aforementioned conditions can be fulfilled in the subsequent production of source and drain regions deliberately for a large number off transistors. In a process according to the invention, a semiconductor substrate is prepared from a first semiconductor material, or, in a layer formed from the semiconductor substrate, at least one linear depression is formed in the surface of the semiconductor substrate, whose width is less than or equal to the gate lengths of the field-effect transistor to be reduced; alternating layers of the first semiconductor material and a second semiconductor material different from the first are repeatedly grown, until the surface is substantially planarized; and, finally, a cluster layer of the second semiconductor material is grown. Already, at the time the first growth of a layer of the second semiconductor material, in the case of SiGe, there occurs, for energy reasons, a certain material accumulation of Ge in the recessed sections and thus a strain field is generated in a Si layer deposited thereon in these regions.
In this manner, by suitable substrate preparation, it is possible to cause clusters to be generated substantially along a line such that during a subsequent process step, the source and drain regions must be formed on both sides of this line.
The field-effect transistor according to the invention can be realized in various semiconductor material systems. A first implementation possibility concerns the silicon/germanium material system. A field-effect transistor made of this material system thus includes a semiconductor substrate made of silicon and a cluster layer in which a plurality of semiconductor clusters made of SiXGe1xe2x88x92X (0xe2x89xa6X less than 1) are arranged on the substrate and are embedded in silicon material.